1. Field of the Invention
The present invention generally relates to the manufacture of microwave/millimeter wave control devices. More particularly, the present invention relates to a novel variable attenuator/modulator for wide-bandwidth applications.
2. Description of the Related Art
High frequency attenuators/modulators are an integral component in a number of communication and radar systems. Variable attenuators, for example, are widely used in automatic gain control circuits. Presently, PIN diodes are used for variable attenuators/modulators. The bandwidth of a PIN modulator, however, is limited by the capacitance of the diode. PIN modulators are therefore typically limited to a bandwidth of 20 GHz. The microwave power a PIN modulator can handle is also limited by the breakdown voltage of the PIN diode, typically less than a few watts. In addition, it is difficult to monolithically integrate PIN diodes with transistors for circuit applications.
Therefore, there is a need for a wide-bandwidth, high-power variable attenuator/modulator which can be integrated with hybrid and monolithic circuits.
Researchers have reported oscillatory indirect magnetic exchange interaction in nearly-free-electron metals, such as copper. See "Oscillatory Magnetic Exchange Coupling through Thin Copper Layers," by S. S. P. Parkin et al., Phys. Rev. Lett., Vol. 66, No. 16, April 1991. In the Physical Review Letters article, antiferromagnetic coupled Co/Cu superlattices were reported to exhibit extraordinarily large saturation magnetoresistances. Giant values of saturation magnetoresistance in antiferromagnetic Co/Cu multilayers containing thin Co and Cu layers were further reported in "Giant Magnetoresistance in Antiferromagnetic Co/Cu Multilayers," by S. S. P. Parkin et al., in Appl. Phys. Lett., Vol. 58, No. 23, June 1991. The Applied Physics Letters article describes that in the presence of a magnetic field, the resistance of a Cobalt/Copper (Co/Cu) superlattice changes from high resistance to low resistance. Although the article discusses the construction of the superlattice structure and suggests the use of the structure in magnetic field sensor devices, no applications using the structure are disclosed.